Polysaccharide conjugation with detoxified E. coli heat labile enterotoxin (LT) used as vaccine

ABSTRACT

A detoxified recombinant  E. coli  heat-labile enterotoxin mutant, LTS61K, is employed as a carrier protein to conjugate polysaccharide. The LTS61K contains a mutated mature sub-unit A (LTA) that includes lysine at amino acid position 61 and a wild-type mature sub-unit B (LTB). Various types of bacterial capsular polysaccharide antigens were chemically conjugated with the LTS61K protein by a reductive amination reaction. The conjugated polysaccharide-LTS61K products were physically, chemically and biochemically identified as soluble form. Rabbits were immunized intramuscularly to determine the immunogenicity of conjugated vaccines by ELISA to detect anti-polysaccharide antigen IgG titers and serum bactericidal assay thereby determining the functional activity of the antibodies. Study results show that conjugated polysaccharide-LTS61K vaccines induce higher polysaccharide-specific IgG titers and greater bactericidal activity in sera than that of polysaccharide alone or polysaccharide mixed with LTS61K. The presence of anti-LTS61K serum IgG antibody alleviates travel diarrhea caused by  E. coli  (ETEC enterotoxigenic  E. coli ).

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed to U.S. Provisional Patent Application Ser. No. 61/330,650, filed on May 3, 2010, the disclosure of which is incorporated herein by reference in its entirety.

SUBMISSION OF SEQUENCE LISTING

The Sequence Listing associated with this application is filed in electronic forma via EFS-Web and hereby incorporated by reference into the specification in its entirety. The name of the text file containing the Sequence Listing is Sequence_Listing_15356_00006. The size of the text filed is 11 KB, and the text file was created on Jul. 18, 2011.

BACKGROUND OF THE INVENTION

Polysaccharide vaccines, when prepared without carrier protein, lack immune memory responses. Currently known conjugated vaccines include bacterial capsular polysaccharides such as Haemophilus influenzae type b, Neisseria meningitidis group C, and Streptococcus pnemoniae serotypes 1, 3, 4, 5, 6B, 7F, 9V, 14, 19F, 23F. These vaccines are covalently conjugated to a carrier protein, such as tetanus toxoid, diphtheria toxoid, CRM197, a mutant nontoxic diphtheria toxin, or Neisseria meningitidis outer membrane protein. These polysaccharide conjugate vaccines could induce T-cell dependent response, especially in infants below the age of two years; and they could prime long term immunologic memory, produce high affinity antibody, and could lower the rate of nasopharyngeal colonization and transmission.

However, the majority of the currently marketed bacterial polysaccharide conjugate vaccines applied tetanus toxoid (TT) or diphtheria toxoid (DT) as a carrier protein. TT and DT, these two toxoid proteins, are regular vaccines for infants/children; high frequency vaccination of TT and DT within a short time could have impact on the immunogenicity and safety. (Reduced response to multiple vaccines sharing common protein epitopes that are administered simultaneously to infants. Infect. Immun. 1998; 66(5):2093-8; Immunogenicity and safety of a combination pneumococcal-meningococcal vaccine in infants: a randomized controlled trial. JAMA 2005; 293(14):1751-8). Therefore, this invention provides a new type of the carrier protein LTS61K for its use on the conjugate vaccine.

SUMMARY OF THE INVENTION

This invention includes polysaccharide conjugation with a detoxified E. coli heat labile enterotoxin (LT) useful as vaccine to protect or immunize against the effects of infectious bacteria such as Haemophilus influenzae and Streptococcus pneumoniae and alleviate travel diarrhea caused by enterotoxigenic E. coli.

One aspect of this invention relates to covalently conjugated polysaccharide-LTS61K vaccines isolated in purified form. These conjugated products have unexpectedly superior immunogenic and bactericidal properties in mammals. Another aspect relates to a method of administering an effective amount of conjugated polysaccharide-LTS61K vaccine to a mammal in need of protection from Haemophilus influenzae type b (Hib). The vaccines of the present invention stimulate T-helper cell response, exhibit strong booster response upon re-exposure and have high antibody titers.

Yet another aspect of the invention relates to the method of producing conjugated polysaccharide-LTS61K by reductive amination and isolating purified conjugated product. In accordance with the present invention, it was discovered that the preferred method of conjugating polysaccharide and LTS61K for making the vaccine of the present invention is periodate oxidation of native PS followed by reductive amination.

The LTS61K employed in the conjugates of the present invention is described in PCT Application Number PCT/US2007/075801, filed Aug. 13, 2007; in U.S. Applications Numbers US11/779,419 filed Jul. 18, 2007, U.S. Ser. No. 12/120,953 filed May 15, 2008, and U.S. Ser. No. 12/729,649 filed Mar. 23, 2010; and in Taiwan Patent Application No. 95139707 filed Oct. 27, 2006 and issued January 2009. The subject matter described in each of these applications is hereby incorporated by reference.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a generalized depiction of a LT carrier protein.

FIG. 2 is the structural representation of Hib PRP saccharides.

FIG. 3 shows several conjugation methods.

FIG. 4 shows a reductive amination method according to the present invention.

FIG. 5 depicts PRP and LTS61K conjugation by reductive amination.

FIG. 6 shows the HPLC-SEC-RI elution profile of oxidized-PRP after NaIO₄ treatment.

FIG. 7 is an NMR spectrum of oxidized-PRP. This confirms the formation of aldehyde groups on PRP after periodate oxidation.

FIG. 8 shows a Far-UV CD spectra that confirms no secondary structure difference between LTS61K and PRP-LTS61K conjugated samples; and also a Fluorescence spectra which confirms no λ max tertiary structure difference between LTS61K and PRP-LTS61K conjugated samples.

FIG. 9 shows amino acid analyses that confirm the successful conjugation of PRP to LTS61K and the formation of a covalent bond.

FIG. 10 shows SDS-PAGE Western Blotting analysis to confirm the covalent conjugation between PRP and LTS61K.

FIG. 11 shows purified PRP-LTS61K conjugates analyzed by IEF PAGE.

FIG. 12 shows IEF Western Blotting to confirm the covalent conjugation between PRP and LTS61K.

FIG. 13 confirms the purity of purified conjugate vaccine via HPLC-SEC-UV-MALLS-RI.

FIG. 14 shows purified PRP-LTS61K conjugates analyzed by IEF PAGE to confirm the purity of purified conjugate vaccine.

FIG. 15 summarizes a rat immunogenicity study of Hib PRP-LTS61K conjugates of the present invention.

FIG. 16 summarizes a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention.

FIG. 17 illustrates results of a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention, results of the rabbit serum bactericidal titer assay and anti-PRP 1gG Ab titers.

FIG. 18 illustrates additional information about the rabbit immunogenicity study.

FIG. 19 illustrates results of a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention, results of the rabbit serum bactericidal titers (BA) and anti-PRP 1gG Ab titers (OD).

FIG. 20 shows results of additional rabbit immunogenicity ELISA of anti-PRP and anti-LTS61K antibodies response studies.

FIG. 21 shows the results of additional rabbit serum bactericidal assay and anti-PRP IgG Ab.

FIG. 22 shows results of additional rabbit serum bactericidal assay and anti-PRP IgG Ab after a fourth immunization.

FIG. 23 shows the immunogenicity in mouse on pneumococcal PS serotype 14-LTS61K conjugates.

DETAILED DESCRIPTION OF THE INVENTION

We have now discovered that conjugated polysaccharide-LTS61K vaccines made in accordance with the present invention surprisingly induce higher polysaccharide-specific IgG antibody titers and greater bactericidal activity in sera than that of polysaccharide or polysaccharide mixed with LTS61K.

A list of abbreviations employed herein is as follows:

-   CFU: colony forming unit -   ELISA: enzyme linked immunosorbent assay -   Hib: Haemophilus influenzae type b -   IEF: isoelectric focusing -   LT: heat labile enterotoxin -   MALLS: multiple angle laser light scattering -   OD: optical density -   PNPS: pneumococcal polysaccharide -   PRP: polyribosyl ribitol phosphate -   PS: polysaccharide -   SDS-PAGE: sodium dodecyl sulfate polyacrylamide gel electrophoresis -   SEC_HPLC: size exclusion high pressure liquid chromatography -   RI: reflective index

The new carrier protein employed in the conjugates of the present invention is detoxified recombinant E. coli heat-labile enterotoxin mutant, LT, more specifically LTS61K. In the LT mutant, the A and B sub-units form a typical AB₅ holotoxin structure. The detoxified LT mutant (LTS61K) contains a mutated mature sub-unit A (LTA) that includes K at amino acid position 61 and a wild-type mature sub-unit B (LTB). LTS61K renders the product significantly less toxic than wild-type LT. The carrier protein is depicted in FIG. 1 of the drawings.

The invention of the LTS61K, which was selected as one of the starting materials for the present invention, is based on the unexpected discovery that an LT containing a mutated LTA exhibits reduced toxicity compared to its wild type counterpart while retaining immunogenicity. This mutated LTA has an amino acid substitution at the position corresponding to position 61 of a wild-type LTA, whose amino acid sequence is shown in SEQ ID NO:1. Accordingly, LTS61K features an isolated polypeptide including a mutated LTA that contains an amino acid residue other than S, T, and F, at the position corresponding to position 61 of SEQ ID NO:1. The substituting amino acid residue can be D, E, H, I, K, L, N, P, Q, R, Y or W. It can be a naturally occurring amino acid or a non-naturally occurring amino acid, e.g., a D-amino acid or a β-amino acid. In one example, the LTA has the amino acid sequence of SEQ ID NO:2, 3, 4 or 5. An LT containing this mutated LTA exhibits reduced toxicity, i.e., <10-5-fold that of a wild-type LT containing SEQ ID NO:1.

In accordance with the present invention, Haemophilus influenzae type B was cultured and its capsular polysaccharide antigen, polyribosylribitol phosphate (PRP), was purified. The PRP is linear, has a negative charge and is hydrophilic. The Hib PRP saccharide with molecular weight 345 and formula 10C, 18H, 11O, 1P is depicted as in FIG. 2 of the drawings.

PRP is conjugated onto LTS61K by a chemical reductive amination reaction employing appropriate molar ratios of polysaccharide PRP to protein (LTS61K) which produce conjugate vaccines. Ranges of molar ratios of PRP:LTS61K are shown in Table 1, below. Preferably, the range of molar ratios of PRP:LTS61K is between about 3:1 and about 60:1, and mole/mole 10₄ ⁻/PRP is between about 0.1 and about 0.4.

TABLE 1 Different ratios of PRP/LTS61K conjugation test IO₄ ⁻/ Average PRP repeat unit PRP/ PRP/ PRP:LT PRP:LT repeat PRP Reac- Protein Protein S61K S61K unit (Colorimetric tion LTS61K LTS61K (w:w) (m:m) (m/m) assay) time (w/w) (m/m) 0.5:1    3:1 0.1 42 2 weeks 0.059 0.68 1:1  6:1 0.1 42 2 weeks 0.099 1.14 1.5:1    9:1 0.1 42 2 weeks 0.13 1.5 2:1 28:1 0.2 17.5 2 weeks 0.21(10/48) 3 3:1 42:1 0.2 17.5 2 weeks 0.23 3.2 4:1 56:1 0.2 17.5 2 weeks 0.24 3.4 3:1 42:1 0.2 17.5 3 weeks 0.21 2.9 3:1 42:1 0.2 17.5 4 weeks 0.35 4.9 3:1 42:1 0.2 17.5 5 weeks 0.33 4.6 3:1 42:1 0.2 17.5 6 weeks 0.31 4.3 3:1 50:1 0.3 14.2 2 weeks 0.34 5.8 3:1 60:1 0.4 12.2 2 weeks 0.37(10/27) 7.4  0.5(10/20)

Further in accordance with the present invention, a variety of different types of the bacterial capsular polysaccharides antigens from Haemophilus influenzae type b and Streptococcus pneumoniae are chemically conjugated with the LTS61K protein by reductive amination reactions. A variety of different molar ratios of NaIO₄:PRP as shown in Table 1, above, were employed to oxidize polysaccharides to produce short lengths of polysaccharides with different repeating units. Following the proper molar ratio of the polysaccharide reacted with LTS61K, the successfully conjugated and purified polysaccharide-LTS61K conjugate products were obtained. The products were in soluble form and were physically and chemically evaluated by SEC-HPLC, orcinol, SDS-PAGE, western blotting, IEF, GM1-binding activity, circular dichroism and fluorescent assays to identify the successful chemical conjugation of the polysaccharide antigen and LTS61K carrier protein.

Example of the Procedure of Making Polysaccharide Conjugated with LTS61K:

A variety of potential conjugation processes are illustrated in FIG. 3 of the drawings. A generalized flow diagram of the reductive amination process selected in accordance with the present invention is found at Glyconjugate J. 1989.6:489 and is reproduced as FIG. 4 of the drawings.

In accordance with the present invention, a polysaccharide is cleaved by periodate oxidation to smaller fragments to produce aldehyde end groups. Thereafter, conjugation of the oxidized polysaccharide to LTS61K protein is carried out by reductive amination. More specifically, an example of the process is as follows:

A. Polysaccharide activation (periodate oxidation): Native polyribosylribitol phosphate (PRP), which is purified capsular polysaccharide of Haemophilus influenzae type b, in an amount of 5 mg/mL is mixed with periodate IO₄ ⁻) 0.3 or 0.6 or 1.2 mg/mL (molar ratio (m/m) of IO₄ ⁻to PRP=0.1, 0.2, 0.4). This mixture was allowed to stand at 4° C. in the dark for 24 hours. Then glycerol was added to terminate the reaction. The resulting oxidized PRP underwent dialysis against ddH₂O using 3.5 K membrane to remove impurities. The resulting product was sterile filtered with 0.22 um filter. The periodate oxidation procedure provided fragmentation of the polysaccharide PRP into different chain lengths. The range of average number of repeating units of PRP was about 40 to 10.

B. Polysaccharide-LTS61K conjugation: Preparation of PRP and LTS61K conjugate, mixture of oxidized PRP obtained from above, purified LTS61K (which was disclosed in the previous obtained/filed patents) and sodium cyanoborohydride (NaBH₃CN), in 10 mg/mL, 3 mg/mL and 10 mg/mL, respectively, the reaction was carried for 2 weeks at 4° C. in the dark. The reaction was terminated by adding sodium borohydride NaBH₄ to quench unreacted aldehydes on PRP, the preparation was then dialyzed against ddH₂O with 10 K membrane.

C. Characterization of the polysaccharide conjugated LTS61K products: Following the proper molar ratio of the polysaccharide reacted with LTS61K, the successfully conjugated and purified polysaccharide-LTS61K conjugate products were obtained. The products with soluble form were physically, chemically and biologically evaluated by SEC-HPLC, Orcinol, SDS-PAGE, Western blotting, IEF, GM1-binding activity, circular dichroism and fluorescent assays to confirm the successful chemical conjugation of the polysaccharide antigen and LTS61K carrier protein, ratio of polysaccharide to LTS61K protein, and its immunogenicity. FIG. 5 of the drawings summarizes reaction conditions and assays employed in accordance with specific exemplification of the present invention.

Reference is now made to the Figures of drawings which pertain to confirmation of structure and purity of the conjugates and vaccine.

FIG. 6 shows the HPLC-SEC-RI elution profile of oxidized-PRP after NaIO₄ treatment, This figure illustrates the process of the invention and results are readily reproducible.

FIG. 7 is an NMR spectrum of oxidized-PRP. This confirms the formation of aldehyde groups on PRP after periodate oxidation.

FIG. 8 shows a Far-UV CD spectra that confirms no secondary structure difference between LTS61K and PRP-LTS61K conjugated samples; and also a fluorescence spectra which confirms no λ max tertiary structure difference between LTS61K and PRP-LTS61K conjugated samples.

FIG. 9 shows amino acid analyses that confirm the successful conjugation of PRP to LTS61K and the formation of a covalent bond.

FIG. 10 shows SDS PAGE Western Blotting analysis to confirm the covalent conjugation between PRP and LTS61K.

FIG. 11 shows purified PRP-LTS61K conjugates analyzed by IEF PAGE.

FIG. 12 shows IEF Western Blotting to confirm the covalent conjugation between PRP and LTS61K.

FIG. 13 confirms the purity of purified conjugate vaccine via HPLC-SEC-UV-MALLS-RI.

FIG. 14 shows purified PRP-LTS61K conjugates analyzed by IEF PAGE to confirm the purity of purified conjugate vaccine.

Table 2, below, is the result of a GM1 binding assay that confirms that LTS61K protein retained its binding activity following conjugation.

TABLE 2 0.2/2 w 0.2/4 w 0.4/2 w 0.4/4 w PRP- PRP- PRP- PRP- LTS61K LTS61K LTS61K LTS61K conjugate conjugate conjugate conjugate GM1-binding 5.19 × 10⁻¹⁰ 4.77 × 10⁻¹⁰ 4.23 × 10⁻¹⁰ 2.79 × 10⁻¹⁰ activity (unit = M) Mammalian pre-clinical studies showing efficacy of the polysaccharide-LTS61K conjugates:

New Zealand White rabbits were immunized intramuscularly three times, at two week intervals, using the intended human dose of the polysaccharide, polysaccharide mixed with LTS61K, or polysaccharide-LTS6K conjugate to evaluate the potency of the conjugate vaccines. All vaccine preparations contained no AlPO4 or other related adjuvant. The immune responses of all vaccines were determined by ELISA to detect anti-polysaccharide antigen specific IgG titers and serum bactericidal assay to detect the functional activity of the antibodies. The study results indicated that only successfully conjugated polysaccharide-LTS61K vaccines could induce higher polysaccharide-specific IgG antibody titers and greater bactericidal activity in sera than that of polysaccharide alone, or polysaccharide mixed with LTS61K only. The animal immunogenicity studies suggest that detoxified E. coli heat-labile enterotoxin holotoxin including LTS61K protein is useful as carrier protein of polysaccharide to significantly stimulate a specific immune response of polysaccharide antigens.

Reference is now made to additional Figures of drawings that pertain to results of initial mammalian studies.

FIG. 15 summarizes a rat immunogenicity study of Hib PRP-LTS61K conjugates of the present invention.

FIG. 16 summarizes a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention.

FIG. 17 illustrates results of a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention, results of the rabbit serum bactericidal titer assay and anti-PRP 1gG Ab titers. Only the PRP-LTS61K conjugates were able to induce anti-PRP IgG Ab titers. The extra added LTS61K into the PRP-LTS61K conjugates did not enhance the anti-PRP IgG Abs.

FIG. 18 illustrates additional information about the rabbit immunogenicity study.

FIG. 19 illustrates results of a rabbit immunogenicity study of Hib PRP-LTS61K conjugates of the present invention, results of the rabbit serum bactericidal titers (BA) and anti-PRP 1gG Ab titers (OD).

Additional studies were conducted employing the procedures and observing the results set forth below:

New Zealand White rabbits were immunized intramuscularly three times and four times, at two week intervals, using the intended human dose of the polysaccharide, polysaccharide mixed with LTS61K, and polysaccharide-LTS61K conjugate to evaluate the potency of the conjugate vaccines. All preparations contained no AlPO₄ or other related adjuvant. The immune responses of all vaccines were determined by ELISA to detect anti-polysaccharide antigen specific IgG titers and serum bactericidal assay to show the functional activity of the antipolysaccharide antibodies. The responses of sera anti-LTS61K IgG titers were also determined by ELISA, and in vitro Caco-2 cell cAMP induction and Y-1 adrenal cell toxicity rounding up study were used to determine the sera anti-LTS61K Ab neutralizing ability with the wild type LT. In vivo rabbit ileal loop challenge studies were conducted to examine the function of the anti-LTS61K antibodies. The animal immunogenicity studies suggest that detoxified E. coli heat-labile enterotoxin holotoxin including LTS61K protein is useful as carrier protein of polysaccharide to significantly stimulate a specific response of polysaccharide antigens. LTS61K, itself, can be used as an immunogen, and, effectively produces antibodies against wild type LT.

A. Rabbit immunogenicity studies. Rabbit immunization intramuscularly using 10 ug dose of PRP, with test articles including: PRP, PRP conjugated with LTS61K and PRP mixed with LTS61K; two and three boosters were given at biweekly intervals. Blood samples were collected at 14 days after each immunization, and collected serum samples were stored at −80° C. until use.

B. Anti-polysaccharide and anti-LTS61K antibodies determined by ELISA. PRP-BSA conjugate was coated on the 96-well plate in the concentration of PRP at 100 ng/well for anti-PRP antibodies determination, and coated with LTS61K at 250 ng/well for the anti-LTS61K antibodies determination. The coated plates were incubated for 16 hours at 4° C., and following incubated with 5% skim milk as blocking buffer for one hour at 37° C. Animal sera were tested starting from a dilution of 1:50. Specific antibodies were measured using a horseradish peroxidate-conjugated goat anti-rabbit IgG incubated for 1 hour at 37° C., and revealed the antibodies by adding TMB (tetramethyl benzidine) peroxidase substrate, and after 10 minutes, the reaction was terminated by the addition of 12% H₂SO₄ and the absorbance was read at OD (optical density) 650-450 nm (reference wavelength 650 nm). The ELISA titers were expressed as the reciprocal of the last dilution which gave OD 450≥0.5.

C. Serum bactericidal assay. Antibody dependent complement mediated bactericidal activity was conducted for the serum diluted titers that kill more than 50% of the Haemophilus influenzae type b (Hib) colonies. Serum samples were pretreated in 56° C. for 30 minutes to inactivate the complement of rabbit serum. Two fold serial dilutions of the serum samples with volume of 10 uL in 96-well U-bottom plates were prepared. Following, added 20 uL of the diluted Hib culture which was prepared in 1000 CFU/20 uL. The mixtures were further incubated at 37° C. with 5% CO₂ for 15 minutes, and added 50 uL of the 1:1 hank's buffer diluted baby rabbit complement into each well, and incubated the mixture at 37° C. with 5% CO₂ for 60 minutes. Plated 5 uL of the baby rabbit complement added mixture in the chocolate agar plates, and incubated the plates at 37° C. with 5% CO₂ for 16 hours. Counted the Hib colony forming units and determined the serum dilution titers that killed more than 50% Hib colonies. All the serum samples are compared with the reciprocal number of dilution titers in graphics.

D. Induction of cAMP study in Caco-2 cells by the neutralized wild type LT with rabbit anti-LTS61K serum antibody was performed, using a cyclic AMP (cAMP) EIA kit (Enzo Life Science). Ten nanograms of wild type LT were incubated with 150 uL anti-LTS61K rabbit serum (1:100 dilution) at room temperature. After one hour incubation, the mixture was added to the Caco-2 cell plate (5×10⁴ cell/well), and Caco-2 cells were further incubated at 37° C. with 5% CO2 for 2 hours. Following, cells were washed with PBS, and lysed with 0.1M HCL (200 uL per well), and neutralized with 0.1M NaOH. The cell lysis products were collected by centrifugation at 660×G for 10 mins at room temperature. The resultant suspensions were assayed for intracellular cAMP levels by utilizing the commercial EIA kit.

E. Neutralization study of wild type LT with serious dilutions of rabbit anti-LTS61K serum was examined on Y-1 mouse adrenal tumor cells. For the Y-1 cell assay, serial two fold dilutions of rabbit sera 50×, 100× to 102,400× were premixed with wild type LT 10⁻⁵ ug (which is the EC50, that the toxin concentration of the wild type LT to produce greater than 50% cell rounding up on Y-1 adrenal cell), following the mixture were added to Y-1 adrenal cell, 5×10⁴ cells/well. The cells were observed for morphological changes (cell rounding up) after 24 hours incubation.

F. Rabbit ileal loop fluid accumulation study: the neutralization of the rabbit anti-LTS61K serum antibody against the wild type LT was conducted in the rabbit ileal loop assay. Rabbit ileal loops with 5.5 cm segments were made on the rabbits after being immunized with PRP or PRP conjugated with LTS61K or PRP mixed with LTS61K. Wild type LT in the concentration ranges from 0.01 to 1 ug were administrated to each study animal; the amount of fluid accumulated in each segment was measured after 18 hours.

Results of the Immunogenicity Effect of Polysaccharide Conjugated with LTS61K: Mammalian Pre-Clinical Studies Showing Efficacy of the Polysaccharide-LTS61K Conjugates

The study results are described below with reference to FIGS. 20 through 23 of the drawings and Tables 3 through 5.

Antibodies to PRP and to LTS61K were determined by ELISA. Results of immunogenicity in rabbits were shown in FIG. 20; only rabbits given PRP-LTS61K conjugates three injections elicit high anti-PRP IgG antibody titers and were 1000-fold greater than PRP or PRP mixed with LTS61K. The major immunoglobulin induced by PRP-LTS61K conjugates vaccine was IgG. The presence of high titer of anti-LTS61K IgG antibody did not interfere with the immune response of anti-PRP IgG Ab.

In FIG. 20, which shows the rabbit immunogenicity ELISA of anti-PRP and anti-LTS61K antibodies response results, all animals received a total of three intramuscular doses each of 10 ug conjugated PRP at 2 week intervals. Sera were collected and assayed at one week post dose 1 (1WPD1), one week post dose 2 (1WPD2) and two weeks post dose 3 (2WPD3) as depicted in FIG. 20.

The protective potential of the anti-PRP antibodies induced by the conjugates were evaluated by testing the bactericidal activity of the rabbit sera. The bactericidal titers were determined against Haemophilus influenzae type b (Hib) Eagen strain, data were presented as reciprocal serum dilution that killed more than 50% of the Hib colonies. FIG. 21 shows that rabbits after three times intramuscular immunization, at two week intervals, in PRP 10 ug per dose, the anti-PRP IgG Abs and bactericidal titers were similar in all of the three batches of in-house prepared PRP-LTS61K conjugates, with the conjugated PRP average repeating units in 40 or 10, and the PRP-LTS61K conjugates doses 10 ug/48 ug to 10 ug/30 ug per dose. Meanwhile, PRP and PRP mixed with LTS61K immunized rabbit sera were unable to kill the Hib bacterial, which reciprocal titers <2, and could not elicit anti-PRP IgG antibodies as well. Only the successful PRP-LTS61K conjugates have a bactericidal effect with the high titers of anti-PRP antibodies.

FIG. 21 shows rabbit serum bactericidal assay and anti-PRP IgG antibodies responses after rabbits received three intramuscular doses each of 10 ug conjugated PRP at 2 week intervals. Tested sera were collected at day 43, two weeks post dose 3.

A re-boosted study was conducted on rabbits after they received a primary-three immunization of the PRP-LTS61K conjugates. The results are summarized in FIG. 22, which demonstrated bactericidal titers and anti-PRP IgG antibodies titers gradually decreased with time, although 6WPD3 (6 weeks post dose 3) rabbit sera bactericidal titers were lower than the rabbit sera 2WPD3, the rabbit sera anti-PRP IgG antibodies and bactericidal titers could be effectively enhanced after a following dose of PRP-LTS61K conjugates. However, these phenomena could not be observed on the PRP or PRP mixed with LTS61K immunized rabbits.

Regarding FIG. 22, rabbits were immunized with different ratios of PRP-LTS61K conjugates. The sera were obtained at different times, and ELISA anti-PRP IgG Abs and bactericidal assays were performed. The results demonstrate correlations between the serum anti-PRP IgG antibodies and the serum bactericidal activities, and that a 4^(th) immunization on rabbits with PRP-LTS61K conjugates effectively re-boosts the anti-PRP Abs.

Table 3 shows that wild type LT toxin at 10 ng was unable to stimulate an increase of intracellular releasing of the cAMP levels in Caco-2 cells, only when rabbit sera with high anti-LTS61K antibodies; those were PRP-LTS61K conjugates and PRP mixed with LTS61K immunized rabbit sera. In contrast, rabbit sera obtained from PRP immunized rabbits did not prevent wild type LT from increasing cAMP levels in Caco-2 cells.

TABLE 3 Induction of cAMP study in Caco-2 cells by the neutralized wild type LT with rabbit anti-LTS61K serum antibody. Rabbit immunized PRP/LTS61K cAMP Samples (ug/ug) (pmol/mL) PRP-LTS61K conjugate 10/45 3.36 PRP-LTS61K conjugate 10/53 2.68 PRP mixed with LTS61K 10 + 45 3.31 PRP 10 19.1 Positive control wt LT 10 ng — 30.89 Negative control PBS — 3.09

A neutralization study of wild type LT with serial dilutions of rabbit anti-LTS61K serum was conducted on Y-1 mouse adrenal tumor cells. The results in Table 4 demonstrate that only those rabbits' sera present with the anti-LTS61K antibodies effectively neutralize the toxicity of the wild type LT, which occurred on PRP-LT61K conjugates and PRP mixed with LTS61K immunized rabbit sera, but not in sera of rabbits immunized with PRP.

TABLE 4 Neutralization Study Rabbit immunized PRP/LTS61K Y-1 adrenal cell Samples (ug/ug) neutralization titer PRP-LTS61K 10/45 3200 X conjugate PRP-LTS61K 10/53 3200 X conjugate PRP mixed with 10 + 45 3200 X LTS61K PRP 10 No neutralization

The data in Table 5 shows that at the concentration of 0.01 ug per 5.5 cm segment ileal loop of wild type LT fluid accumulation was induced in the rabbit with no anti-LTS61K antibodies; however, in the rabbit with anti-LTS61K antibodies, there was no fluid accumulation. These results confirm those previously obtained in vitro Caco-2 cAMP induction and Y-1 adrenal cell neutralization studies, that serum anti-LTS61K antibodies are able to neutralize the wild type LT. Severe hemorrhagic lesions on ileum mucosa were observed on the rabbit without anti-LTS61K Ab administered with wild type LT in 0.5 and 1.0 ug per segment.

TABLE 5 Rabbit ileal loop fluid accumulation study. Rabbit no.1 with anti- Rabbit no.2 with anti- Rabbit without anti- LTS61K Ab LT Ab LTS61K Ab Fluid Fluid Wild type LT Fluid Lesions on accumula- Lesions accumula- Lesions on dose accumulation ileum tion (mL/ on ileum tion (mL/ ileum (ug/segment) (mL/segment*) mucosa** segment) mucosa segment) mucosa 0 0.0 — 0.0 — 0.0 — 0.01 4.95 — 0.0 — 0.0 — 0.1 8.8 + 0.0 — 4.4 — 0.5 10.45 ++ Not Not 11.0 — done done 1 12.1 +++ 11.0 + 11.0 — *Fluid accumulation is measured in each 5.5 cm segment of rabbit ileal loop. **Severe hemorrhagic lesions on ileum mucosa were observed on the rabbit without anti-LTS61K Ab administered wild type LT in 0.5 and 1.0 ug per segment.

The results of an animal immunogenicity study as an example of pneumococcal polysaccharide serotype 14 covalent conjugated with LTS61K is shown in FIG. 23. Antibody titers to PNPS serotype 14 were determined by ELISA. Samples were PNPS14-LTS61K conjugates with PS average repeating units of 94 or 40, PNPS14 alone, PNPS14 mixed with LTS61K, and PBS. Only the conjugated products were able to induce high anti-PNPS14 IgG antibodies. The presence of the chemical adjuvant, aluminum hydroxide, during the immunization program did not benefit or significantly enhance the anti-PNPS14 antibodies.

FIG. 23 shows the immunogenicity in mice on pneumococcal PS serotype 14-LTS61K conjugates. Only animals given PNPS14-LTS61K conjugates elicit high anti-PS IgG antibody titers and were 500-fold greater than PNPS14 or PNPS14 mixed with LTS61K.

Many modifications and alterations of the present invention will become apparent to those skilled in the art without departing from the spirit and scope of the present invention which is defined by the claims. 

What is claimed is:
 1. A conjugate comprising a polysaccharide and a detoxified E. coli heat labile enterotoxin (LT), wherein the LT is a holotoxin comprising a mutant protein LT61 that exhibits GM-1 binding activity and comprises the wild type amino acid sequence of SEQ ID NO: 1 but with a lysine amino acid residue at a position corresponding to position 61 in SEQ ID NO: 1 to form the mutant protein LTS61K having reduced toxicity compared to the wild type amino acid sequence while retaining immunogenicity, the polysaccharide being configured with aldehyde end groups and being covalently conjugated to the LT holotoxin through reductive amination to form the conjugate as a soluble product in which the mutant protein LT61 retains GM-1 binding activity, wherein the polysaccharide is a bacterial capsular polysaccharide antigen from Haemiophilus influenzae type b or Streptococcus pneumonia.
 2. The conjugate of claim 1 where the bacterial capsular polysaccharide is polyribosylribitol phosphate.
 3. An immunogenic composition for administration to mammals which comprises the conjugate of claim
 1. 4. An immunogenic composition comprising the conjugate of claim 1, wherein the immunogenic composition does not comprise an adjuvant.
 5. The conjugate of claim 1, wherein the conjugate consists of the polysaccharide and the detoxified E. coli heat labile enterotoxin (LT).
 6. The conjugate of claim 1, wherein the polysaccharide is cleaved into fragments by periodate oxidation.
 7. The conjugate of claim 1 wherein the polysaccharide is a bacterial capsular polysaccharide antigen from Streptococcus pneumonia. 